The present invention relates to an automatic stabilizer unit for free trolleys and attached load carriages in an inverted power and free conveyor system.
Power and free conveyor systems for moving bulky items through a manufacturing or assembly plant are well known. In a power and free conveyor, there are a power and a free conveyor track, generally disposed vertically with respect to each other. The "power" track is generally an endless chain with pusher members periodically inserted within the chain. These pusher members are closely confined within the power track and each one includes a driving "dog" which is oriented to engage a "free" trolley. The dog may or may not be selectively retractable.
A plurality of free trolleys travel within the "free" or carrier track which follows the same path as the power track but is spaced vertically relative thereto. At least some of the free trolleys include a driving member which can selectively engage the dog on a corresponding pusher member within the power track. The original power and free conveyor systems were suspension systems with the power track disposed above the free track. Loads to be carried on the conveyor system were suspended beneath the free trolleys in the free track. These suspension systems have reached a high degree of sophistication and can include features such as the ability to stop and accumulate free trolleys in specific accumulating areas and transfer zones which include intersections where loads can be transferred between conveyor systems.
More recently, in response to the specific requirements of the automobile industry, floor mounted or "inverted" power and free systems have been developed. In these inverted systems, the power track and the free track are disposed beneath the floor of the factory, with the free track positioned above the power track. A plurality of load carriages are attached to the free trolleys through a slot in the floor. Each load carriage is usually attached to two or more free trolleys with the load carriage being disposed above the floor and driven along the conveyor path by associated free trolleys.
These inverted systems have the capability of handling bulkier and heavier loads, such as automobile chassis, while minimizing many dangerous conditions found in suspension systems. For example, inverted systems allow workers to safely climb on and off of the load carriages and they eliminate the danger inherent in the swinging loads of suspension systems.
At the same time, the development of inverted systems has presented a new and unique series of problems to designers. In a large factory, a single power and free conveyor can run for a mile or more. In a conveyor of this length there may be literally hundreds of power members and associated free trolleys and load carriages. In such a system, even a small amount of unnecessary drag in each trolley and carriage has an enormous cumulative effect on the system. Thus, even the provision of free castering stabilizing wheels on each load carriage would add an unacceptable amount of drag to the system. Thus, load carriages are often connected directly to the trolleys via shafts extending through the slot in the floor and disposed along the center line of the load carriages with the load carriages themselves not contacting the floor at all.
However, manufacturing facilities, such as automobile assembly plants, which use inverted power and free systems often include numerous automated assembly stations such as robotic welders and riveters. At such stations, alignment between the automobile chassis to be welded and the robotic equipment is critical, often With tolerances within thousandths of an inch. At such points, it is absolutely necessary for the load carriages in the conveyor to be stabilized. This is particularly true since, at these same stations, it is common for workers to climb onto and off of the carriages, thus introducing destabilizing forces which are multiplied by the moment arm presented by any displacement of the worker's position from the centerline of the carriages.
A designer, then, is faced with a dilemma. Stabilizing wheels which contact the floor surface under the outboard load components of a carriage add an unacceptable amount of drag to a conveyor system, but stabilization is needed when the load carriages and loads are positioned at critical assembly line stations such as robotic welders and riveters.
It is clear then, that a need exists for a stabilizing system for an inverted power and free conveyor which can be selectively engaged to provide stabilization for the loads and load carriages as they approach conveyor stations which include robotic assembly stations or the like with consequent close stabilization tolerance requirements. At the same time, such a stabilizing system should not include carriage wheels which constantly contact the floor surface, since the cumulative drag from wheels on all of the load carriages would be unacceptable.